1. Field of the Invention
This invention relates to an optical system for detecting the state of focus, and in particular to such optical system suitable for determining the focusing of a single lens reflex camera such as a camera or a video camera.
2. Description of the Prior Art
The heretofore proposed optical systems for detecting the state of focus of single lens reflex cameras or the like are generally classified into two types, i.e. the image sharpness detection type and the image displacement detection or image correlation detection type.
In the image sharpness detection system such as disclosed in, for example, Japanese Laid-open Patent Application No. 155308/1980, the degrees of sharpness of the object images on three CCD line sensors, disposed on a predetermined image-forming plane of the image-forming optical system and equidistantly forwardly and rearwardly thereof respectively, are compared to detect the state of focus.
On the other hand, the optical system for detecting the state of focus of a camera using the image displacement detection or image correlation detection system is known, for example, from Japanese Laid-open Patent Application No. 143315/1978, Japanese Laid-open Patent Application No. 7323/1979 or U.S. Pat. No. 4,185,191. FIG. 1 of the accompanying drawings schematically show the construction of a device using the system of U.S. Pat. No. 4,185,191. An image forming light beam l from an image-forming lens, not shown, passes through a partly transmitting quick return mirror 1 and then enters a submirror 2 and is downwardly reflected and enters a lenslet array comprising juxtaposed lenslets 3a, 3b, . . . . Respective sets of light-receiving sensors 4a.sub.1, 4a.sub.2 ; 4b.sub.1, 4b.sub.2 ; . . . are disposed rearwardly of the respective lenslets 3a, 3b, . . . . In the case of such image displacement detection or image correlation detection system, the lenslets 3a, 3b, . . . are present at positions conjugate with the predetermined image-forming plane and therefore, during the in-focus state, entirely the same information light enters the sets of sensors 4 a.sub.1, 4a.sub.2 ; 4b.sub.1, 4b.sub.2 ; . . . disposed rearwardly of the respective lenslets 3a, 3b, . . . When the image-forming plane of the image-forming light beam l does not exist on the lenslet array lying on the predetermined image-forming plane, that is, during the out-of-focus state, the sensor array comprising sensors 4a.sub.1 and 4b.sub.1 and the sensor array comprising sensors 4a.sub.2 and 4b.sub.2 receive image information having an amount of image displacement corresponding to the amount of defocus. In FIG. 1, the quick return mirror 1 is not an ordinary beam splitter using a semi-transparent film of dielectric material, but a so-called wavefront division type patterned beam splitter having light-transmitting portions 1a of diameter W. Such a patterned beam splitter performs the role of a low-pass filter, and the size and distribution of the light-transmitting portions 1a are determined in conformity with the arrangement pitch p of each sensor 4a.sub.1, 4b.sub.1 of the sensor array.
When it is supposed that such optical system is incorporated as the image displacement detection or image correlation detection system into a camera, there is, for example, a method whereby the image-forming light beam l is divided into two light beams and two image intensity distributions regarding the object images formed by the divided two light beams are formed and used to detect the state of focus. A roof type submirror 5 as shown, for example, in FIG. 2 of the accompanying drawings is conceivable as a beam splitting element for dividing the image-forming light beam l into two light beams. In FIG. 2, the light beam l from an image-forming lens 6 enters the roof type submirror 5 having two reflecting surfaces 5a and 5b, and then is reflected by the reflecting surfaces 5a and 5b to provide light beams la and lb, which are incident on downwardly positioned line sensors 7a and 7b each comprising, for example, a CCD consisting of a plurality of sensors. The outputs of the line sensors 7a and 7b may then be compared to thereby detect the state of focus. Where the image-forming light beam 1, as shown in FIG. 3 of the accompanying drawings, enters as a circle R intermediate the reflecting surfaces 5a and 5b of the roof type submirror 5, equal quantities of light are directed to the line sensors 7a and 7b, but when use is made of an image-forming lens of great F-number, the off-axis light beam l', for example, enters the roof type submirror 5 as a circle R' and as a result, almost all of the off-axis light beam l' enters the reflecting surface 5b to provide a reflected light 1b' and therefore, there occurs a phenomenon that the reflected light beam enters one line sensor 7b while it hardly enters the other line sensor 7a. In a condition in which imbalance has thus occurred between the quantities of light entering the line sensors 7a and 7b, the distributions of quantity of light regarding the image intensity distributions entering the line sensors 7a and 7b when the object to be photographed is a surface of uniform brightness are expressed as schematically shown by A and B in FIG. 4 of the accompanying drawings. In FIG. 4, the abscissa is coincident with the arrangement direction x of the line sensors 7a and 7b shown in FIG. 2 and 0x is the position whereat the light beams on the optical axes on the respective line sensors 7a and 7b intersect each other. The ordinate represents the relative quantity of light, A indicates the distribution of quantity of light on the line sensor 7a which is rightwardly downward, and B indicates the distribution of quantity of light on the line sensor 7b which is rightwardly upward. Such a phenomenon that the distribution of quantity of light is not uniform even if the surface of uniform brightness is imaged is tentatively called the shading, and for example, in the focus detecting device of FIG. 2, if said shading is present when the distributions of quantity of light A and B of the images created on the line sensors 7a and 7b are to be compared and operationally processed, the state of focus cannot be detected properly.